Page 111 of the Goodheart--Willcox automotive encyclopedia, The Goodheart-Willcox Company, Inc., South Holland, Ill., 1979 describes that as fuel is burned in an internal combustion engine, about one-third of the heat energy in the fuel is converted to power. Another third goes out the exhaust pipe unused, and the remaining third must be handled by a cooling system. This third is often underestimated and even less understood.
Most internal combustion engines employ a pressurized cooling system to dissipate the heat energy generated by the combustion process. The cooling system circulates water or liquid coolant through a water jacket which surrounds certain parts of the engine (e.g., block, cylinder, cylinder head, pistons). The heat energy is transferred from the engine parts to the coolant in the water jacket. In hot ambient air temperature environments, or when the engine is working hard, the transferred heat energy will be so great that it will cause the liquid coolant to boil (i.e., vaporize) and destroy the cooling system. To prevent this from happening, the hot coolant is circulated through a radiator well before it reaches its boiling point. The radiator dissipates enough of the heat energy to the surrounding air to maintain the coolant in the liquid state.
In cold ambient air temperature environments, especially below zero degrees Fahrenheit, or when a cold engine is started, the coolant rarely becomes hot enough to boil. Thus, the coolant does not need to flow through the radiator. Nor is it desirable to dissipate the heat energy in the coolant in such environments since internal combustion engines operate most efficiently and pollute the least when they are running relatively hot. A cold running engine will have significantly greater sliding friction between the pistons and respective cylinder walls than a hot running engine because oil viscosity decreases with temperature. A cold running engine will also have less complete combustion in the engine combustion chamber and will build up sludge more rapidly than a hot running engine. In an attempt to increase the combustion when the engine is cold, a richer fuel is provided. All of these factors lower fuel economy and increase levels of hydrocarbon exhaust emissions.
To avoid running the coolant through the radiator, coolant systems employ a thermostat. The thermostat operates as a one-way valve, blocking or allowing flow to the radiator. FIG. 2 of U.S. Pat. No. 4,545,333 shows a typical prior an thermostat controlled coolant systems. Most prior an coolant systems employ wax pellet type or bimetallic coil type thermostats. These thermostats are self-contained devices which open and close according to precalibrated temperature values.
Coolant systems must perform a plurality of functions, in addition to cooling the engine parts. In cold weather, the cooling system must deliver hot coolant to heat exchangers associated with the heating and defrosting system so that the heater and defroster can deliver warm air to the passenger compartment and windows. The coolant system must also deliver hot coolant to the intake manifold to heat incoming air destined for combustion, especially in cold ambient air temperature environments, or when a cold engine is started. Ideally, the coolant system should also reduce its volume and speed of flow when the engine pans are cold so as to allow the engine to reach an optimum hot operating temperature. Since one or both of the intake manifold and heater need hot coolant in cold ambient air temperatures and/or during engine start-up, it is not practical to completely shut off the coolant flow through the engine block.
Practical design constraints limit the ability of the coolant system to adapt to a wide range of operating environments. For example, the heat removing capacity is limited by the size of the radiator and the volume and speed of coolant flow. The state of the self-contained prior art wax pellet type or bimetallic coil type thermostats is controlled solely by coolant temperature. Thus, other factors such as ambient air temperature cannot be taken into account when setting the state of such thermostats.
Numerous proposals have been set forth in the prior art to more carefully tailor the coolant system to the needs of the vehicle and to improve upon the relatively inflexible prior art thermostats. Several of these prior art systems are described in co-pending U.S. application Ser. No. 08/390,711 which is identified above and incorporated herein by reference.
The above referenced co-pending related applications disclose a unique temperature control system for controlling the flow of temperature control fluid in an internal combustion engine. These co-pending applications also discuss in detail the effect that cold temperatures have on the oil in an engine. Specifically, when the temperature of the oil in an engine falls below approximately 190 degrees Fahrenheit, sludge begins to develop which contaminates the oil. This typically occurs in prior art thermostatic engines during start-up and warm-up. During these periods of operation, the coolant temperature rises more rapidly than the internal engine temperature. Since the thermostat is actuated by coolant temperature, it often opens before the internal engine temperature has reached its optimum value, thereby causing coolant in the water jacket to prematurely cool the engine. As a result, a cold running engine will have less complete combustion in the engine combustion chamber and will build up sludge more rapidly than a hot running engine.
In co-pending U.S. application Ser. No. 08/306,240, a novel electronic engine temperature control valve (hereafter, "EETC valve") is disclosed which controls the flow of the temperature control fluid through the engine so as to maintain the engine at or near its optimum temperature. One embodiment of the EETC valve disclosed in that application utilizes hydraulic oil from the oil pan to actuate the valve.
When the hydraulic oil flowing in the fluid lines to and from the valve is relatively hot, such as during normal engine operation, the actuation of the valve is relatively quick and smooth. However, when the oil in the lines begins to cool, such as after the engine is shut-off, the viscosity of the oil will increase resulting in slower and less smooth actuation of the valve. This adversely affects the operation of the valve.
A need therefore exists for an system which minimizes the hydraulic fluid remaining within the lines leading to a hydraulically operated valve in an engine after the engine has been shut-off.